Power transmission shaft

ABSTRACT

A power transmission shaft which can be evaluated on torsional fatigue strength based on a management indicator different from compressive residual stress in an outer surface. In a power transmission shaft having outer circumferential serrations  1   a,    1   b , torsional fatigue strength is evaluated by using compressive residual stress at a depth of 30 μm (and 50 μm) from an outer surface of a rising portion of each concave of the outer circumferential serrations  1   a,    1   b  as a management indicator. A high torsional fatigue strength can be secured by having a compressive residual stress of 1150 MPa or more at a depth of 30 μm (and 50 μm) from the outer surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power transmission shaft used in an apparatus such as automobiles and industrial machines.

2. Description of the Related Art

A power transmission shaft used in an automobile or an industrial machine has been requested to attain weight reduction and strength enhancement. Regarding strength enhancement, static strength and torsional fatigue strength enhancement has been demanded. An example of power transmission shafts which aim to enhance static strength and torsional fatigue strength is disclosed in Japanese Unexamined Patent Publication No. 2003-307,211. In the power transmission shaft disclosed in this publication, shot peening is applied to parts to be improved in torsional fatigue strength. Shot peening imparts compressive residual stress and improves torsional fatigue strength.

Japanese Patent No. 3,374,667 discloses that torsional fatigue strength can be improved securely by increasing compressive residual stress in an outer surface to, for instance, 850 MPa or more by shot peening. Furthermore, this Japanese patent discloses that double shot peening is carried out in order to increase compressive residual stress in the outer surface. The double shot peening means applying shot peening twice under different conditions. Namely, conventionally, shot peening has been applied to increase torsional fatigue strength, and compressive residual stress in an outer surface has been used as a management indicator.

SUMMARY OF THE INVENTION

By the way, there have been demands for a further enhancement of torsional fatigue strength and at the same time for enhancement of torsional fatigue strength by a simpler process such as applying shot peening once.

The present invention has been conceived under these circumstances. It is an object of the present invention to provide a power transmission shaft which can be evaluated on torsional fatigue strength based on a management indicator different from compressive residual stress in an outer surface.

The present inventors have earnestly studied and made a lot of trials and errors in order to attain this object. As a result, the present inventors have found that it is effective in improving torsional fatigue strength to use, as a management indicator, compressive residual stress not in an outer surface but in a part at a predetermined depth from an outer surface and have completed the present invention.

A power transmission shaft of the present invention is a power transmission shaft having an axial notch on its outer circumferential surface and the axial notch has an axial end having a compressive residual stress of 1150 MPa or more at a depth of 30 μm from an outer surface. It is more preferable that the axial end has a compressive residual stress of 1150 MPa or more at a depth of 50 μm from the outer surface.

The present invention has the following advantages. According to the power transmission shaft of the present invention, compressive residual stress in a part at a depth of 30 μm from an outer surface is used as a first management indicator. The use of compressive residual stress in a part at a depth of 30 μm from an outer surface as a management indicator permits appropriate evaluation of torsional fatigue strength. Besides, torsional fatigue strength can be improved sufficiently by having a compressive residual stress of 1150 MPa or more in a part at a depth of 30 μm from an outer surface.

For example, when a power transmission shaft which has compressive residual stress of 1200 MPa or more in an outer surface and less than 1150 MPa in a part at a depth of 30 μm from the outer surface is compared in torsional fatigue strength with a power transmission shaft which has a compressive residual stress of less than 1200 MPa in an outer surface and 1150 MPa or more in a part at a depth of 30 μm from the outer surface, the latter has been proven to have a higher torsional fatigue strength. This example demonstrates that it is more appropriate to use, as a management indicator, a compressive residual stress in a part at a predetermined depth from an outer surface rather than a compressive residual stress in an outer surface.

Moreover, according to the power transmission shaft of the present invention, compressive residual stress in a part at a depth of 50 μm from an outer surface is used as a second management indicator. The use of compressive residual stress in a part at a depth of 50 μm from an outer surface in addition to the above first management indicator permits evaluation of a higher torsional fatigue strength. Besides, torsional fatigue strength can be improved further by having compressive residual stress of 1150 MPa or more in parts at depths of 30 μm and 50 μm from an outer surface.

Furthermore, the following advantages can also be obtained by using compressive residual stress in a part at a predetermined depth from an outer surface as a management indicator. In some cases, shot peening is applied in order to impart compressive residual stress to a power transmission shaft. When single shot peening is applied, that is, shot peening is carried out once, a part having a maximum compressive residual stress is not an outer surface but a part at a depth of not less than 10 μm, for instance, from an outer surface. Therefore, even when compressive residual stress in the outer surface is not so high, a sufficient torsional fatigue stress can be secured by having a compressive residual stress of 1150 MPa or more in a part at a predetermined depth (e.g., 30 μm, 50 μm) from the outer surface.

On the other hand, if compressive residual stress in an outer surface alone is used as a management indicator as before, there arises a need to apply double shot peening, i.e., carry out shot peening twice when compressive residual stress in the outer surface after single shot peening is below a reference value. Therefore, even when sufficient torsional fatigue strength is secured, an appropriate judgment cannot be made on the base of the conventional management indicator and, in some cases, double shot peening is applied.

In contrast, the use of compressive residual stress in a part at a predetermined depth from an outer surface as a management indicator makes it possible to determine without faults cases where single shot peening is sufficient and double shot peening is unnecessary. Namely, according to the present invention, torsional fatigue strength which is more than the conventional can be secured by a simpler process than before (single shot peening, for instance)

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings.

FIG. 1 is an axial cross-sectional view of a drive shaft for an automobile.

FIGS. 2(a) and 2(b) are partial cross-sectional views of an outer circumferential serration 1 a.

FIG. 3 is a graph showing a residual stress distribution by depth from an outer surface of rising portions 1 c of outer circumferential serrations 1 a, 1 b.

FIG. 4 shows results of a torsional fatigue test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a power transmission shaft, an axial notch, and a process of imparting compressive residual stress according to an embodiment of the present invention will be described in detail.

(1) Power Transmission Shaft

The power transmission shaft according to the embodiment of the present invention can be used in an apparatus such as automobiles and industrial machines, as mentioned above. A typical example of power transmission shafts used in automobiles is a power transmission shaft for an automobile connected to a universal joint. For instance, the power transmission shaft is an intermediate shaft both ends of which are respectively connected to universal couplings such as constant velocity joints. The power transmission shaft is made of a metal material such as an iron-based material which includes iron as a main component.

In the power transmission shaft, thermal treatment can be usually applied to at least an axial notch. After the thermal treatment is applied, compressive residual stress is imparted to an axial end of the axial notch. This imparting of compressive residual stress is conducted by shot peening, for instance, as will be mentioned later. Namely, in the power transmission shaft, thermal treatment is applied to the aforementioned axial notch, before shot peening, for instance. Owing to the thermal treatment, at least a sufficient static strength can be secured. Examples of the thermal treatment include induction hardening, carburizing, nitriding and flame hardening.

(2) Axial Notch

The axial notch according to the embodiment of the present invention can be a concave portion when seen in an axial cross section. This concave portion is formed, in some cases, on the entire circumference and, in other cases, on a part of the circumference. For example, when the power transmission shaft is an automotive power transmission shaft to be connected to a universal joint, an axial notch, which is a concave portion, formed on the entire circumference is a part to be assembled with a boot for covering the universal joint. An axial notch formed on a part of the circumference is a part to be connected to or assembled with the universal joint, such as an outer circumferential serration and key grooves.

The power transmission shaft having such an axial notch has a remarkably low torsional fatigue strength particularly at an axial end of the axial notch. By imparting compressive residual stress, torsional fatigue stress of the axial end of the axial notch can be enforced. The term ‘the axial end of the axial notch’ as used herein means a portion rising from a bottom of a concave to an outer circumferential surface. For instance, when the axial notch is a concave formed on the entire circumference, the axial end is each of the axial ends of the concave. When the axial notch is a concave of a serration, formed on a part of the circumference, the axial end is a rising portion of each concave of the serration.

(Process of Imparting Compressive Residual Stress)

Imparting of compressive residual stress according to the embodiment of the present invention can be carried out by shot peening. For instance, this shot peening is a process of pelting the aforementioned axial end at a speed of 55 to 90 m/sec with shots having an average particle radius of one-third to two-thirds of a minimum curvature radius of the abovementioned axial end and a higher hardness than a surface hardness of the abovementioned axial end before shot peening by 50 to 300 Hv. This shot peening can securely impart a compressive residual stress of 1150 MPa or more to a part at a predetermined depth from an outer surface.

When the axial end is a rising portion of each concave of a serration, a part having the abovementioned minimum curvature radius is each end of a concave bottom of the serration when seen in a radial cross section of the rising portion of each concave of the serration. The outer surface hardness before the shot peening means an outer surface hardness of the power transmission shaft before shot peening is applied.

By using shots having an average particle radius of not more than two-thirds of a minimum curvature radius of the axial end, a part having the minimum curvature radius can securely be pelted with the shots, and accordingly compressive residual stress can securely be imparted to this part. Moreover, by using shots having an average particle radius of not less than one-third of a minimum curvature radius of the axial end, a compressive residual stress of 1150 MPa or more can be imparted to a part at a predetermined depth from an outer surface. It is more preferable that the average particle radius of the shots is close to two-thirds of the minimum curvature radius of the axial end. In this case, a high compressive residual stress can be imparted. Namely, higher torsional fatigue strength can be obtained.

Now, the present invention will be described in detail by way of preferred embodiments.

(Structure of Automotive Drive Shaft)

An intermediate shaft used in a drive shaft for an automobile will be described as a preferred embodiment of the power transmission shaft of the present invention. First, the drive shaft for an automobile will be described with reference to FIG. 1. FIG. 1 is an axial cross-sectional view of the drive shaft for an automobile. As shown in FIG. 1, the drive shaft for an automobile comprises an intermediate shaft 1, an inboard joint 2, an outboard joint 3 and boots 4, 5.

(1.1) Intermediate Shaft 1

The intermediate shaft 1 is a power transmission shaft formed of a solid rod. The intermediate shaft 1 transmits power which has been input from a driving shaft side of the inboard joint 2 to a driven shaft side of the outboard joint 3. Formed on outer circumferential surfaces of both ends of the intermediate shaft 1 are outer circumferential serrations 1 a, 1 b which are in parallel with an axial direction.

Now, the outer circumferential serration 1 a formed on an end portion of the intermediate shaft 1 will be described in detail with reference to FIGS. 2(a) and 2(b). FIG. 2(a) is an axial cross-sectional view of the end portion having the outer circumferential serration 1 a. FIG. 2(b) is a cross-sectional view taken along line B-B of FIG. 2(a), namely, an enlarged view of a rising portion 1 c of one concave of the outer circumferential serration 1 a.

As shown in FIG. 2(a), each concave of the outer circumferential serration 1 a formed on the intermediate shaft 1 has the rising portion (an axial end) 1 c on an axial center side of the intermediate shaft 1 (the right side of FIG. 2). This rising portion 1 c is a slant portion which gradually rises from a concave bottom, which forms the outer circumferential serration 1 a, toward the outer circumferential surface. As shown in FIG. 2(b), each concave (axial notch) of the outer circumferential serration 1 a including the rising portion 1 c has a radially cross sectional shape comprising side portions constituted by involute curves and a concave bottom. Of each concave, a part having a minimum curvature radius is each end portion 1 d of the concave bottom when seen in a radial cross section. It is to be noted that the outer circumferential serration 1 b formed on the other end portion of the intermediate shaft 1 has almost the same shape as the above mentioned outer circumferential serration 1 a formed on the one end portion of the intermediate shaft 1.

Moreover, grooves (axial notches) 1 e, 1 f for attaching the boots 4, 5 are formed near the outer circumferential serrations 1 a, 1 b on the axially center side of the intermediate shaft 1. These grooves 1 e, 1 f for attaching the boots 4, 5 have concave shapes when seen in an axial cross-sectional direction and are formed on the entire circumference. Each of these grooves 1 e, 1 f is a portion to be engaged with and fixed to one end of each of the boots 4, 5, which will be mentioned later.

(1.2) Inboard Joint 2

The inboard joint 2 is a constant velocity joint such as a slidable tripod joint. This inboard joint 2 is connected to a power input side of the intermediate shaft 1, as shown in FIG. 1. This inboard joint 2 comprises a tripod-type inner member 11, tripod-type rollers 12 and a tripod-type outer member 13.

The tripod-type inner member 11 comprises a boss portion 21 and three trunnions 22. The boss portion 21 has a roughly cylindrical shape and an inner circumferential serration 21 a on its inner circumferential surface. This inner circumferential serration 21 a of the boss portion 21 is fitted on and engaged with the outer circumferential serration 1 a formed on the one end portion of the intermediate shaft 1. Namely, the tripod-type inner member 11 is integrally connected to the intermediate shaft 1. Each of the trunnions 22 has a roughly cylindrical shape and is disposed so as to extend from the outer circumference of the boss portion 21 in a radially outward direction.

Each of the tripod-type rollers 12 has a roughly cylindrical shape with an outer circumferential surface in a partially spherical shape. Each of the tripod-type rollers 12 is disposed on the outer circumferential surface of each of the trunnions 22 of the tripod-type inner member 11 so as to be freely rotatable.

The tripod-type outer member 13 comprises a driving shaft portion 31, and a tubular portion 32 which has a tubular shape with a bottom and is formed integrally with the one end portion of the driving shaft portion 31 so as to extend in a tubular shape. Formed on an inner circumferential surface of the tubular portion 32 are three guide grooves 32 a which are in parallel with the axial direction. The tripod-type rollers 12 are respectively engaged with these guide grooves 32 a in the circumferential direction and are disposed in the guide grooves 32 a so as to be freely slidable in the axial direction. Moreover, formed on an outer circumferential surface of the tubular portion 32 is a groove 32 b to be engaged with and fixed to the other end of the boot 4, which will be mentioned later. The groove 32 b for attaching the boot 4 has a concave shape when seen in an axial cross section and is formed on the entire circumference.

(1.3) Outboard Joint 3

The outboard joint 3 is a constant velocity joint such as a fixed ball joint. This outboard joint 3 is connected to a power output side of the intermediate shaft 1, as shown in FIG. 1. This outboard joint 3 comprises a ball-type inner member 41, a ball-type outer member 42, a cage 43 and balls 44.

The ball-type inner member 41 has a roughly cylindrical shape. The outermost circumferential surface 41 a of this ball-type inner member 41 has uniformly circular arc shapes when seen in axial cross sections, namely, has a partially spherical shape. Formed on an outer circumferential surface of the ball-type inner member 41 are inner ball guide grooves 41 b which are in parallel with the axial direction and comprise six circular arc concaves disposed at even distances when seen in a radial cross section. Formed on an inner circumferential surface of the ball-type inner member 41 is an inner circumferential serration 41 c. This inner circumferential serration 41 c of the ball-type inner member 41 is fitted on and engaged with the outer circumferential serration 1 b formed on the other end portion of the intermediate shaft 1. Namely, the ball-type inner member 41 is integrally connected to the intermediate shaft 1.

The ball-type outer member 42 comprises a driven shaft 51 and a tubular portion 52 which has a tubular shape with a bottom and is formed integrally with one end portion of the driven shaft 51 so as to extend in a tubular shape. The innermost circumferential surface 52 a of the tubular portion 52 has uniformly circular arc shapes when seen in axial cross sections, namely, has a partially spherical shape. Formed on an inner circumferential surface of the tubular portion 52 are outer ball guide grooves 52 b which are in parallel with the axial direction and comprise six circular arc concaves disposed at even distances when seen in a radial cross section. Moreover, formed on an outer circumferential surface of the tubular portion 52 is a groove 52 c for attaching the other end portion of the boot 5, which will be mentioned later. This groove 52 c for attaching the boot 5 has a concave shape when seen in an axial cross section and is formed on the entire circumference.

The cage 43 has a roughly cylindrical shape and is disposed between the ball-type inner member 41 and the tubular portion 52 of the ball-type outer member 42. An inner circumferential surface of the cage 43 is formed in a shape corresponding to that of the outermost circumferential surface 41 a of the ball-type inner member 41. On the other hand, an outer circumferential surface of the cage 43 is formed in a shape corresponding to that of the innermost circumferential surface 52 a of the tubular portion 52 of the ball-type outer member 42. Namely, the cage 43 is relatively rotatable without making contact with the ball-type inner member 41 or the ball-type outer member 42. Moreover, the cage 43 has six holes at even distances.

The balls 44 are respectively engaged in a circumferential direction with the inner ball guide grooves 41 b of the ball-type inner member 41 and the outer ball guide grooves 52 a of the tubular portion 52 of the ball-type outer member 42 so as to be freely rotatable. In addition, the balls 44 respectively penetrate the circular holes of the cage 43. Namely, the balls 44 transmit rotation of the ball-type inner member 41 to the ball-type outer member 42.

(1.4) Boots 4, 5

The boot 4 has a bellows shape. One end portion of the boot 4 is fixed to the groove 1 e of the intermediate shaft 1 for attaching the boot 4, and the other end portion of the boot 4 is fixed to the groove 32 b of the inboard joint 2 for attaching the boot 4. On the other hand, the boot 5 has a bellows shape. One end portion of the boot 5 is fixed to the groove 1 f of the intermediate shaft 1 for attaching the boot 5, and the other end portion of the boot 5 is fixed to the groove 52 c of the outboard joint 3 for attaching the boot 5.

2) Process of Producing the Intermediate Shaft 1

Now, a process of producing the above mentioned intermediate shaft 1 will be described. First, the outer circumferential serrations 1 a, 1 b and the grooves 1 e, 1 f for attaching the boots 4, 5 are formed on a raw material comprising a roughly rod-shape steel. For example, the outer circumferential serrations 1 a, 1 b are formed by rolling, and the grooves 1 e, 1 f for attaching the boots 4, 5 are formed by lathe turning.

Second, thermal treatment comprising induction hardening is applied to the overall axial length of the intermediate shaft 1. In parts having the outer circumferential serrations 1 a, 1 b and the grooves 1 e, 1 f for attaching the boots 4, 5 of the intermediate shaft 1, t/r≈0.45 to 0.5, where r is the radius of the shaft and t is the depth from the surface to be heat treated. Third, shot peening is applied on the parts having the outer circumferential serrations 1 a, 1 b and the grooves 1 e, 1 f for attaching the boots 4, 5 of the intermediate shaft 1. The shot peening applied on the outer circumferential serrations 1 a, 1 b will be described in detail later. Induction hardening improves static strength and fatigue strength, and additional application of shot peening further enhances torsional fatigue strength.

(3) Shot Peening

Next, shot peening to be applied on the parts having the outer circumferential serrations 1 a, 1 b of the intermediate shaft 1 has been conducted under a variety of conditions, and resultant torsional fatigue strength in each case has been evaluated. The shape and hardness of the intermediate shaft 1 before shot peening, conditions of shot peening, compressive residual stress distribution after shot peening, torsional fatigue test, torsional fatigue strength evaluation results will be described hereinafter.

(3.1) Shape and Hardness of the Intermediate Shaft 1 before Shot Peening

First, the intermediate shaft 1 before shot peening will be described. The outer circumferential serrations 1 a, 1 b of the intermediate shaft 1 have a serration module of 1.05833. The rising portions 1 c of these serrations 1 a, 1 b have a minimum curvature radius of 0.15 mm. As mentioned before, a part having the minimum curvature radius is each end portion 1 d of each concave bottom of the serration 1 a, as shown in FIG. 2(b). The rising portion 1 c of each concave of the serrations 1 a, 1 b have an outer surface hardness of 700 Hv before shot peening. Namely, the outer surface hardness before shot peening is controlled to 700 Hv by applying thermal treatment before shot peening. The thermally treated intermediate shaft 1 has a compressive residual stress of about 230 MPa in the outer surface, and as the depth from the outer surface gets greater, the compressive residual stress gets gradually smaller.

(3.2) Shot Peening Conditions

Next, shot peening conditions will be described. The shot peening conditions were of six kinds from Case 1 to Case 6, as shown in Table 1. As shown in Table 1, these six kinds of conditions are different in the kind of shots and arc height. To be concrete, these six kinds of conditions are different in the material, average particle size (diameter) and average hardness of shots, and arc height. TABLE 1 Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Shots Material steel amorphous steel steel amorphous steel alloy alloy Average Particle Size (μm) 200 200 100 100 50 50 Average Hardness (Hv) 789 925 789 560 925 789 Arc Height (mmN) 0.51 0.515 0.295 0.415 0.19 0.17 Amount of Shots (kg/min) 9.2 same as same as same as same as same as on the left on the left on the left on the left on the left Shot Angle of Impact (°) 90 same as same as same as same as same as on the left on the left on the left on the left on the left Pelting Distance (mm) 150 same as same as same as same as same as on the left on the left on the left on the left on the left Number of Workpiece Revolutions (rpm) 30 same as same as same as same as same as on the left on the left on the left on the left on the left Air Pressure (MPa) 0.3 same as same as same as same as same as on the left on the left on the left on the left on the left Injection Type Direct same as same as same as same as same as Pressure on the left on the left on the left on the left on the left Type Peening Time (sec) 10 same as same as same as same as same as on the left on the left on the left on the left on the left Number of Peening Directions (unit) 1 same as same as same as same as same as on the left on the left on the left on the left on the left Coverage (%) not less same as same as same as same as same as than 100 on the left on the left on the left on the left on the left

As shown in Table 1, the material of shots of Cases 1, 3, 4, 6 is steel and that of Cases 2, 5 is an amorphous alloy. The average particle size (diameter) of shots of Cases 1, 2 is 200 μm, that of Cases 3, 4 is 100 μm and that of Cases 5, 6 is 50 μm. Namely, the average particle radius of shots of Cases 1, 2 is two-thirds of a minimum curvature radius of the rising portions 1 c, that of Cases 3, 4 is one-third of the minimum curvature radius of the rising portions 1 c, and that of Cases 5, 6 is one-sixth of the minimum curvature radius of the rising portions 1 c.

The average hardness of shots of Cases 1, 3, 6 is 789 Hv, that of Cases 2, 5 is 925 Hv, and that of Case 4 is 560 Hv. Namely, the average hardness of shots of Cases 1, 3, 6 is higher than the outer surface hardness before shot peening by 89 Hv, that of Cases 2, 5 is higher than the outer surface hardness before shot peening by 225 Hv, and that of Case 5 is lower than the outer surface hardness before shot peening by 140 Hv.

The amount of shots, shot angle of impact, pelting distance, the number of workpiece revolutions, air pressure, injection type, peening time, the number of peening directions, and coverage were all kept constant, as shown in Table 1. Herein, the shot angle of impact is an angle to the axis of the intermediate shaft 1. The number of workpiece revolutions is the number of revolutions of the intermediate shaft 1 as a workpiece to be shot peened. It is to be noted that although the speed of pelting shots is determined based on air pressure and an average particle size of shots, the speed of pelting shots under all six kinds of conditions ranges from 55 to 90 m/sec.

(3.3) Compressive Residual Stress after Shot Peening

Measurement was conducted on compressive residual stress in the rising portions 1 c of the outer circumferential serrations 1 a, 1 b of the intermediate shaft 1 after the above shot peening. To be concrete, the measurement was conducted on compressive residual stress against depth from the outer surface of the rising portions 1 c. The measurement of compressive residual stress was conducted by digging the rising portions 1 c from the outer surface toward the axis center by electropolishing and measuring compressive residual stress at each predetermined depth from the outer surface by an X-ray stress analyzer. For comparison, measurement was also conducted on compressive residual stress in the rising portions 1 c of the outer circumferential serrations 1 a, 1 b of the intermediate shaft 1 before the above shot peening was applied.

Measurement results are shown in FIG. 3. FIG. 3 shows a residual stress distribution by depth from the outer surface of the rising portions 1 c of the outer circumferential serrations 1 a, 1 b. A negative sign of residual stress on the vertical axis means compressive residual stress.

As shown in FIG. 3, Case 1 had a compressive residual stress of 1115 MPa in the outer surface and a maximum compressive residual stress of about 1370 MPa at a depth of about 45 μm from the outer surface. Case 2 had a compressive residual stress of 1046 MPa in the outer surface and a maximum compressive residual stress of about 1296 MPa at a depth of about 35 μm from the outer surface. Case 3 had a compressive residual stress of 1087 MPa in the outer surface and a maximum compressive residual stress of about 1434 MPa at a depth of about 17 μm from the outer surface.

Case 4 had a compressive residual stress of 558 MPa in the outer surface and a maximum compressive residual stress of about 1080 MPa at a depth of about 20 μm from the outer surface. Case 5 had a compressive residual stress of 1417 MPa in the outer surface and a maximum compressive residual stress of about 1449 MPa at a depth of about 7 μm from the outer surface. Case 6 had a compressive residual stress of 1156 MPa in the outer surface and a maximum compressive residual stress of about 1388 MPa at a depth of about 9 μm from the outer surface. The intermediate shaft 1 before shot peening had a compressive residual stress of 227 MPa in the outer surface and had a smaller compressive residual stress as the depth from the outer surface got greater.

(3.4) Torsional Fatigue Test

The following torsional fatigue test was conducted on the intermediate shafts 1 which were subjected to the shot peening under the abovementioned conditions and the intermediate shaft 1 which was not subjected to shot peening. The torsional fatigue test was conducted by holding the outer circumferential serrations 1 a, 1 b of both the ends of the intermediate shaft 1 and applying a torque of 700 Nm while alternating rotational directions. This application of torque was continued until the intermediate shaft 1 caused failure. The number of times the direction of rotation (torsion) for applying torque was changed was counted until the intermediate shaft 1 caused failure. Of the intermediate shaft 1, a part causing failure is generally the rising portion 1 c of each concave of the outer circumferential serrations 1 a, 1 b.

(3.5) Torsional Fatigue Strength Evaluation Results

The measurement results of the abovementioned torsional fatigue test will be described with reference to FIG. 4. FIG. 4 shows measurement results of the torsional fatigue test. Namely, FIG. 4 shows the number counted until the intermediate shaft 1 caused failure under each kind of shot peening conditions. As shown in FIG. 4, Case 1 showed the counted number of 1,170,499, Case 2 showed that of 750,349, Case 3 showed that of 537,282, Case 4 showed that of 232,965, Case 5 showed that of 123,495, and Case 6 showed that of 104,044. The intermediate shaft 1 which was not shot peened showed that of 138,427. It is to be noted that as the counted number is greater, torsional fatigue strength is higher.

Namely, as apparent from FIG. 4, the intermediate shafts 1 which were subjected to shot peening of Cases 1 to 3 had higher compressive torsional strength than those which were subjected to shot peening of Cases 4 to 6 or the intermediate shaft 1 which was not subjected to shot peening.

Now, torsional fatigue strength will be evaluated with reference to FIG. 3, which shows a compressive residual distribution by depth from the outer surface of the rising portion 1 c of each concave of the outer circumferential serrations 1 a, 1 b, in addition to FIG. 4, which shows the measurement results of the torsional fatigue test.

Shot peening of Cases 1, 2, which showed the highest and second-highest torsional fatigue strength, imparted high compressive residual stress to parts even at great depths from the outer surface. Shot peening of Case 3, which showed the third highest torsional fatigue strength, imparted a higher compressive residual strength to parts even at great depths from the outer surface than shot peening of other cases, although not as high as shot peening of Cases 1, 2. It is to be noted that all of Cases 1 to 3 imparted compressive residual stress of about not less than 1000 MPa in the outer surface. On the other hand, shot peening of Cases 4, 5, which didn't show very high torsional fatigue strength, imparted high compressive residual stress of 1000 MPa or more in the outer surface but low compressive residual stress at a depth of 30 μm or more from the outer surface.

In more detailed analysis, a remarkable difference is seen on compressive residual stress at depths of 30 μm and 50 μm from the outer surface between Cases 1 to 3 and Cases 4 to 6. Namely, Case 1 showed the compressive residual stress of about 1300 MPa both at a depth of 30 μm and at a depth of 50 μm. Case 2 showed the compressive residual stress of about 1250 MPa at a depth of 30 μm from the outer surface and about 1230 MPa at a depth of 50 μm from the outer surface. Case 3 showed the compressive residual stress of about 1230 MPa at a depth of 30 μm from the outer surface and about 400 MPa at a depth of 50 μm from the outer surface. On the other hand, Cases 4 to 6 showed the compressive residual stress of not more than 1000 MPa at depths of 30 μm and 50 μm from the outer surface.

The above analysis has led to the following three findings. First, when compressive residual stress at a depth of 30 μm from the outer surface is 1150 MPa or more, that is to say, a compressive residual stress distribution curve passes below the first reference point in FIG. 3, torsional fatigue strength is high. Second, when compressive residual stress at a depth of 50 μm from the outer surface is 1150 MPa or more in addition to the above first finding, that is to say, a compressive residual stress distribution curve passes below the second reference point in FIG. 3, torsional fatigue strength is much higher. Third, when compressive residual stress in the outer surface is 1000 MPa or more in addition to the above first and second findings, high torsional fatigue strength is secured.

It can also be said from the above analysis that an appropriate shot peening process for producing the intermediate shaft 1 having a high torsional fatigue strength is to pelt parts having the outer circumferential serrations 1 a, 1 b at a speed of 55 to 90 m/sec with shots having an average particle radius of one third to two-thirds of a minimum curvature radius of the rising portion 1 c of each concave of the outer circumferential serrations 1 a, 1 b and a higher hardness than a surface hardness of the rising portion 1 c of each concave of the outer circumferential serrations 1 a, 1 b before shot peening by 50 to 300 Hv.

This invention may be practiced or embodied in still other ways without departing from the spirit or essential characters thereof. The preferred embodiment described herein is therefore illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all variations which come within the meaning of the claims are intended to be embraced therein. 

1. A power transmission shaft having an axial notch on its outer circumferential surface, said axial notch having an axial end having a compressive residual stress of 1150 MPa or more at a depth of 30 μm from an outer surface.
 2. The power transmission shaft according to claim 1, wherein said axial end has a compressive residual stress of 1150 MPa or more at a depth of 50 μm from the outer surface.
 3. The power transmission shaft according to claim 1, wherein said compressive residual stress is imparted by shot peening comprising pelting said axial end at a speed of 55 to 90 m/sec with shots having an average particle radius of one-third to two-thirds of a minimum curvature radius of said axial end and a higher hardness than a surface hardness of said axial end before shot peening by 50 to 300 Hv.
 4. The power transmission shaft according to claim 3, wherein thermal treatment is applied to said axial end before said shot peening.
 5. The power transmission shaft according to claim 2, wherein said compressive residual stress is imparted by shot peening comprising pelting said axial end at a speed of 55 to 90 m/sec with shots having an average particle radius of one-third to two-thirds of a minimum curvature radius of said axial end and a higher hardness than a surface hardness of said axial end before shot peening by 50 to 300 Hv.
 6. The power transmission shaft according to claim 5, wherein thermal treatment is applied to said axial end before said shot peening.
 7. The power transmission shaft according to claim 1, wherein said axial notch is a serration.
 8. The power transmission shaft according to claim 1, wherein said power transmission shaft is a power transmission shaft for an automobile to be connected to a universal coupling, and said axial notch is a part to be connected to said universal coupling. 